When engineers fix a printed circuit board in an electronic device to a metal casing by using a metal screw, a metal spacer having screw threads inside is pressed into (or welded to) a metal casing and the printed circuit board is fixed via that spacer to the metal casing by using a metal screw, as in a widely accepted method, which is shown in FIG. 1.
For example, FIG. 1 discloses a printed circuit board insulation structure for an electronic device that fixes a printed circuit board 31 to a metal casing 32 by using a printed circuit board fixation screw 34 after the engineer in advance pressed a metal spacer 33 into the metal casing 32 (or after welding the metal spacer 33 to the metal casing 32) as disclosed also in Patent Document 1 below.
Note that a sectoral wiring pattern prohibition area 36 is set in a prescribed range from the printed circuit board fixation screw 34 in the printed circuit board 31 in consideration of an insulation distance and a design margin defined by a standard.
Also, a technique of fixing a printed circuit board 41 to a metal casing 42 by using a metal screw via a commercially available insulation support 43 similarly to the technique in FIG. 1 is conventionally known for a case when an engineer desires to fix the printed circuit board 41 to the metal casing 42 while insulating them from each other, as shown in FIG. 2.
In the printed circuit board insulation structure shown in FIG. 1 above, the printed circuit board fixation screw 34 and the metal casing 32 have the same electric potential. The metal casing 32 is connected to for example the frame ground (FG) in order to fix the electric potential.
In this situation, an insulation distance and the wiring pattern prohibition area 36 for a design margin need to be set for the outer periphery of the washer of the screw 34 around the printed circuit board fixation screw 34, the insulation distance being based on a standard defined by a safety standard mainly for electric appliances in the US, authorized by IEC (International Electro-technical Commission) and UL (Underwriters Laboratories Inc.).
In particular, when the operation voltage for a printed circuit board is high, the wiring pattern prohibition area 36 becomes very large, reducing the wiring pattern area and the component mounting area and also making the electronic device larger, which is problematic.
Also, in the method of fixing the printed circuit board 41 to the metal casing 42 in the conventional printed circuit board insulation structure shown in FIG. 2, the commercially available support 43 itself is relatively large, leading to a larger electronic device large, which is problematic.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2012-200071